1. Field of the Invention
The present invention relates to an optical disk, and more particularly, to an optical disk for a mobile device, which is less likely to be deflected.
2. Description of the Related Art
Recently, mobile devices using a small-sized recording medium, such as digital still cameras, portable music players, and personal digital assistants (PDAs), have come into wide use. Small-sized memory cards can be used for such mobile devices, but their relatively high price makes them less attractive to customers than their cheaper counterparts, such as CD-R/RW disks. General apparatuses for recording data on and reproducing data from a CD or DVD, however, are relatively big to be carried, and thus it is necessary to develop a small-sized mobile device for recording data on or reproducing data from a CD or DVD and a disk having a new format appropriate for such a small-sized mobile device.
A conventional high-density optical disk is manufactured by recording data on a substrate having a thickness of 1.1 mm and depositing an optical transmission layer on the top surface of the substrate to a thickness of 0.1 mm using a spin coating or sheet bonding technique.
FIGS. 1A and 1B are a perspective view and a cross-sectional view, respectively, of a conventional optical disk 10 bent due to a process of forming an optical transmission layer. Referring to FIGS. 1A and 1B, an optical disk 10 has an outer diameter Dout of 120 mm, an inner diameter Din of 15 mm, and a thickness t of 1.1 mm. An optical transmission layer 13 formed on the surface of a substrate 11 has a thickness I of 0.1 mm. In FIG. 1B, light perpendicularly incident to the optical transmission layer 13 is refracted after passing through the optical transmission layer 13 and reflected from an interface between the optical transmission layer 13 and the substrate 11, and then exits from the optical disk 10. As shown in FIG. 1B, the exiting light forms a predetermined angle θ (referred to as deflection angle) with the entering light. The conventional optical disk 10 has a deflection angle of 0.2° and is affected by a contractile-force-per-meter of 27 N/m.
As described above with reference to FIGS. 1A and 1B, even though an optical disk having a thickness of 1.1 mm is bent due to a process of forming an optical transmission layer, it is possible to normally drive the optical disk using a general apparatus for recording data on or reproducing data from an optical disk as long as a deflection angle of the optical disk does not exceed a standardized limit for deflection angle, which is 0.7°. However, in order to manufacture an optical disk for a mobile device, a thin substrate having a thickness of 1.1 mm or below is necessary. Since a thinner substrate is more vulnerable to a contractile force generated during the plasticization of an optical transmission layer deposited on the surface of the substrate, the substrate may be bent more severely and provide a larger deflection angle than the substrate 10 shown in FIGS. 1A and 1B. If an optical disk has a deflection angle larger than predetermined degrees, it may be impossible to normally record data on or reproduce data from the optical disk.